contact lens materials
Modulus, Water Content and Dk in Silicone Hydrogels, Part 2
BY LORETTA B. SZCZOTKA-FLYNN, OD, MS, FAAO

In part 1 of this article (August 2006), I explored the relationship between modulus and water content in silicone hydrogel contact lenses. The relationship is fairly linear — as water content goes up, modulus goes down. Does the Dk/water relationship follow the same path? That is, does theory dictate that higher-water silicone hydrogels must have lower Dk values?

I explored this issue with another expert in our field, Philip Morgan, PhD, MCOptom, who is director of Eurolens Research at the University of Manchester in the United Kingdom.

Hydrogel Relationship

For hydrogel contact lens materials, there is an intrinsic relationship between water content and oxygen permeability. Dr. Morgan together with Professor Nathan Efron measured the oxygen permeability and water content of 17 hydrogel contact lens brands and published the relationship as a simple exponential formula. The accurate prediction given by this exponential formula also confirms that the hydrogels can be considered to be part of one material family.

Figure 1. Dk vs. water content for silicone hydrogels.

What about Silicone Hydrogels?

Dr. Morgan explains: "Until the launch of the silicone hydrogen contact lenses, soft lens manufacturers could increase oxygen performance only by adding various components to soft contact lens materials to elevate equilibrium water content, which results in a maximum oxygen transmissibility (Dk/t) in the low 30s — short of the values required for extended wear.

"We recently measured oxygen permeability values for five silicone hydrogel materials (Figure 1). Whereas hydrogel materials exhibit a very tight relation-
ship between water content and permeability (green dots in Figure 1) this isn't the case for silicone hydrogels. Although a general relationship exists between these two parameters (with lower-water-content materials offering higher oxygen performance as these lenses have higher concentrations of silicone moieties), it's not possible to accurately predict oxygen permeability from water content values for silicone hydrogels."

Not All the Same

Dr. Morgan says, "the silicone hydrogen contact lens materials currently on the market are sufficiently different from each other as to be considered from different material families. Similarities exist between materials from the same manufacturer where a close relationship exists between water content and permeability, but this doesn't apply for all the lens brands considered together. For silicone hydrogel contact lenses, clinicians will need to refer to independently published values or to manufacturer-supplied data for oxygen permeability."

Dr. Szczotka-Flynn is an associate professor at Case Western Reserve University Dept. of Ophthalmology and is director of the Contact Lens Service at University Hospitals of Cleveland.